I understand that SD can have either field first, so if you are starting with source that is UFF then just keep it that way.
Edit
I can type faster than read!
I suppose you put the project properties to what the majority of your footage is if it is for SD. If for HD then use UFF.
Supposing if it isn't as simple as 90% SD or HD? Maybe it's more like 60:40?
Project settings are "mostly" for Previewing sanity, yes? If that's the case then what comes next, render, is mostly the issue. Once here I render to whatever I need to deliver in. This last stage shouldn't be an issue.
Don't worry about Project's field order setting. The most important thing is to set the correct field order for all media files (LFF for SD, UFF fo HD). And set the desired deinterlace option (Blend or Interpolate) - this is required for scaling (eg. Pan/Crop) etc. That's it. Vegas will do the rest. You can select any render template you want.
Yes,
so long as the field order is correctly flagged when it comes to rendering Vegas will take care of it correctly.
Preview with mixed media on the T/L I've found to be not so easy going. If you plan on doing a lot of intense editing it may pay to convert all your media to the one format / codec and edit that with your project properties matched to the media. I've never done this, instead just lived with the less than ideal playback experience. YMMV.
This is one of the cases where I would set the project the same as the output format, esp. if there will be generated media, titles, etc.
Field order in the project is irrelevant, as Vegas handles it all in the render.
The only real disadvantage is a hit in preview performance, but G has a nice fast machine now, I understand.
;?)